1. Field of the Invention
This invention relates to a method for producing a thermoplastically moldable fiber-reinforced semifinished product from a mixed nonwoven containing thermoplastic fibers and reinforcing fibers.
2. Background Art
Thermoplastically moldable semifinished products containing reinforcing fibers, in particular glass fibers, are being used to an increasing extent for the production of moldings, in particular for automotive parts. Such “plastic panels” are characterized by high strength and toughness. GMT semifinished products are manufactured on a large scale industrially by combining continuous glass fiber strand mat and molten thermoplastic films in a double band press. This procedure consumes a substantial amount of energy, because the viscous melt must be pressed into the mat at pressures far above 1 bar. It is thus exceptionally difficult, in practice, to achieve a fiber content greater than 45 wt % and an areal weight below 2000 g/m2 by this method. Since the reinforcing fibers in the reinforcing fibers in the glass mats are generally in the form of fiber bundles or “strands”, impregnation with thermoplastic is never entirely complete and uniform, and therefore microscopically heterogeneous regions are present, thus resulting in a high standard deviation in the mechanical properties. This is also the case with thermally expanded GMT, which, due to the restoring forces of the glass fibers needled together, contains air pores which are irregularly distributed within the matrix.
German Patent Application DE-A 36 14 533 describes a method for producing molded articles of thermoplastics which contain a reinforcing insert. Based on textile fiber technology, a nonwoven blend of thermoplastic fibers and reinforcing fibers is produced by carding or air-laying methods and is consolidated, for example, by needling. Cut sections of this nonwoven blend are heated and pressed directly to form three-dimensional molded articles without prior consolidation into a semi-finished product. Complete impregnation is very difficult to obtain, especially with components having a complex shape, so that the mechanical properties of the moldings leave much to be desired.
According to WO 98/3508, in a complex method, blended strands of reinforcing fibers and thermoplastic fibers are first produced and then a nonwoven is produced from them. This nonwoven is pressed on a double band press at high temperature and high pressure to form a semifinished product. Production of mixed strands of reinforcing fibers and thermoplastic fibers is difficult due to the differing tensile elongations and modulus of the different fibers, and only a limited selection of blends is commercially available.
WO 02/062563 describes a continuous method for producing thermoplastically moldable thin semifinished products of a thermoplastic and long reinforcing fibers by dry blending thermoplastic fibers and reinforcing fibers to form a nonwoven blend; consolidating the fiber blend by needling; heating the consolidated nonwoven blend; compressing, employing a calender or a pair of pinch rollers to form a semifinished product, and optionally, laminating a functional layer thereto. Sheet products produced in this manner are dense and have surface irregularities such as waviness.
WO 02/076711 describes a method similar to that of WO 02/062563 for producing thick nonwoven blends, wherein the step of compressing can also be performed by a laminating device at pressures between 1 bar and 10 bar. However, it has been found that at such high pressures the air pores are forced almost completely out of the softened nonwoven blend and the melt flows apart in length and width, resulting in an uncontrolled variation in areal weight and in distortion of the semifinished product, with the result that the boundaries of the semifinished product are wavy rather than smooth and straight.
Similar problems arise in the methods according to EP-B 593 716 and U.S. Pat. No. 4,978,489 in which the nonwoven blend is compression molded by pressure rollers facing each other. In this process, the mat is compressed so strongly that the resulting semifinished product contains maximally only 20 vol %, and preferably 10 to 15 vol % of air pores. In the method according to U.S. Pat. No. 4,948,661 a mixed nonwoven is compressed between heated plattens or a double band press until the air is completely eliminated from the consolidated product.
The compression of mixed nonwovens in calenders or by pressure rollers has the further disadvantage that only low production speeds can be used and that a bulge is formed by the abrupt compression in the gap between rollers, which may result in strong distortion and even the formation of holes.
WO 03/086725 describes an apparatus and a process for making fiber-reinforced composites involving molding a mixed nonwoven mat in a continuous compression belt at relatively high pressure, up to 30 bar. “Pseudo-foamed composite sheets” are said to result from this process, but the air pore content is necessarily very low, and the high pressure will cause distortion of the non-woven and non-homogenous pore distribution. The sheet materials have a very high thermoplastic content, and thus strength and modulus are relatively low.
Published German Application DE-A 195 20 477 discloses a fiber reinforced sheet or panel that is thermally expanded and thus contains air pores. As discussed further below, these air pores are very irregularly distributed in the panel, whereas the air pores in the semifinished product of the present invention are uniformly distributed. The difference can easily be recognized in SEM pictures. Example 1, shows an expanded panel whose weight per unit of area is much greater than 2000 g/m2. The length of the glass fibers is 100 mm, and the content of glass fibers is 30 wt %.
Published German EP-A-758577 discloses a stampable sheet prepared by a papermaking process wherein reinforcing fibers, thermoplastic fibers, and optionally non-fibrous thermoplastic particles are deposited on a foraminous screen from dispersion in water, dried, heated above the thermoplastic melt temperature, and then pressed in a cold press to form a dense stampable sheet. This sheet is then heated agin and allowed to freely expand by a factor of 1.1 to 15. A high pressure, for instance 5 kgf/cm2 (about 5 bar) in Example 1 of the publication, is used to produce the dense intermediate.